Mice homozygous for the autosomal recessive mutation, SCID (SCID mice) are severely deficient in mature lymphocytes. Early B and T lineage-committed cells are clearly present in SCID lymphopoietic tissues, but are unable to differentiate into functional B and T lymphocytes. This developmental arrest is believed to reflect a defect in the system responsible for the recombination of V (variable), D (diversity) and J (joining) gene elements that code for immunoglobulin (Ig) and T cell receptor (TCR) variable regions. This proposal deals with two levels of complexity in the manifestation of the SCID defect. The first concerns whether V(D)J recombination in SCID cells is equally defective at all Ig and TCR loci. Since the recombination of V(D)J elements within and between different Ig and TCR loci follows an ordered developmental pattern, the applicant will test for possible differences in the effect of SCID (or expression of SCID) in lymphocytes representing distinct stages of early development. Different populations of lymphocytes will be analyzed for VDJ recombinase activity and V(D)J recombination, with emphasis on the frequency of recombination and the structure of the recombination junctions at different loci. The second level of complexity concerns the leaky SCID phenotype. This refers to SCID mice that develop a few clones of functional B and T cells and produce detectable serum Ig (Ig+); interestingly, leaky SCID mice with B or T cells only have not been observed. Dr. Bosma will test whether SCID leakiness reflects a low frequency of somatic reversion at the SCID locus, and also possibly, a low frequency of normal Ig and TCR gene rearrangement mediated by the defective SCID recombinase activity. He will also test to what extent the invariable presence of T cells in serum Ig+ SCID mice reflects T cell dependent growth requirements of B cells and/or somatic reversion before the commitment to the B and T cell lineages. These aims will involve testing various cell lines from leaky SCID mice for reversion to normal x-irradiation sensitivity (increased radiosensitivity is a property of SCID cells), analyzing a large sample of B cell hybridomas and/or plasmacytomas from old leaky SCID mice for normal (or abnormal) rearrangement at the non-productive Ig alleles, and constructing athymic nude-SCID mice and chimeric SCID mice. The experiments outlined in this proposal should tell us more about the effect of SCID on early lymphocyte development. This is important for understanding the genetic control of both normal and abnormal lymphocyte differentiation.